1. Field of the Invention
The present invention relates to an elbow prosthesis comprising at least one humeral component and one ulnar component.
2. Background of the Invention
As a function of the articular clearance and of the mode of coupling the humeral and ulnar prosthetic elements together, two principal types of prostheses are distinguished:
prostheses with hinge arrangements, in which a common hinge pin is introduced in aligned orifices provided on the humeral component and on the femoral component. Taking into account the mode of assembling these prostheses, the hinge pin which defines the articulating surface necessarily presents a rectilinear generatrix, which is substantially different from the natural articulating surface of the trochlea. These hinge arrangements present good stability and allow a rotation in the sagittal plane of the joint. However, they prevent transverse movements such as the varus-valgus movement or a movement of ulnar axial rotation. To allow such a movement, the hinge must be provided with a considerable radial clearance, which leads to premature wear of the articulating surfaces.
sliding prostheses, as known in particular from U.S. Pat. No. 4,242,758, in which the ulnar component comes into abutment on a substantially cylindrical humeral articulating surface. Such prostheses allow transverse movements, but are unstable perpendicularly and/or parallel to the sagittal plane, hence there is a considerable risk of dislocation or transverse instability inducing parasitic movements when the elbow is flexed.
It is a particular object of the present invention to overcome these drawbacks by proposing an elbow prosthesis which allows certain transverse movements, such as the varus-valgus movement, while presenting a largely increased stability over known sliding prostheses.
To that end, the invention relates to an elbow prosthesis comprising a humeral component forming a first, substantially cylindrical articulating surface and an ulnar component forming a second articulating surface adapted to be arranged around a part of this first articulating surface and to pivot about a longitudinal axis of this first articulating surface, characterized in that the ulnar component is provided with means for mounting a locking element forming a third articulating surface, extending the second articulating surface and adapted to be disposed around the first articulating surface, these second and third articulating surfaces extending together, in transverse section of the first articulating surface, over more than 180xc2x0 about the first articulating surface.
Thanks to the invention, the second and third surfaces of the ulnar component and of the locking element make it possible to surround or to enclose the first articulating surface, this eliminating the risks of dislocation of the joint. The geometry of the articulating surfaces may be chosen to be close to the natural articulating surfaces and, in particular, is not limited to the cylindrical surfaces with rectilinear generatrix used up to the present time in hinge-type prostheses. In effect, the cylindrical nature of the first articulating surface means that it is generated by rotation of a generatrix about an axis, its generatrix not necessarily being rectilinear but may be concave, which makes it possible to approach the natural morphology of the trochlea best.
According to advantageous aspects of the invention, the prosthesis incorporates one or more of the following features:
The second and third articulating surfaces extend together over an angle included between 190xc2x0 and 360xc2x0, preferably between 225xc2x0 and 315xc2x0, preferably still of the order of 270xc2x0, about the first articulating surface.
The prosthesis comprises means for adjusting, as a function of the patient""s morphology, the position of the longitudinal axis of the first surface with respect to the longitudinal axis of an anchoring rod of the humeral component. In effect, the trochlea may be more or less forward in the sagittal plane with respect to the longitudinal axis of the humerus, independently of the size of the bone, this morphological variation not, up to the present time, being taken into account by elbow prostheses. The prosthesis of the present invention therefore makes it possible to take this variable into account when placing a prosthesis. In particular, the first surface may be provided to be formed by an elongated piece while tabs fast with the anchoring rod are provided with bores for receiving an assembly pin, the relative position of the bores of the piece and of the tabs being adapted as a function of the patient""s morphology. For example, the prosthesis may comprise a plurality of elongated pieces and/or a plurality of rods, these pieces and rods being adapted to be assembled together and allowing different positions of the longitudinal axis of the substantially cylindrical parts with respect to the axes of the rods, to be obtained. According to another variant embodiment, the rod and the elongated piece forming the first surface may be provided to be fixed with respect to each other, in particular in one piece, the prosthesis comprising a plurality of such piece/rod assemblies of different geometries for a given size of humerus. It is then possible for the surgeon, when placing a prosthesis, to choose a rod/piece assembly as a function of the patient""s morphology.
The ulnar element and the locking element are each provided with a free edge, the distance between these free edges being smaller than the minimum diameter of the first articulating surface when the locking element is mounted on the ulnar component. In this way, the assembly formed by the ulnar component and the locking element cannot be dislocated with respect to the first articulating surface.
The assembly means comprise at least one tapping for receiving a screw engaged in a housing provided in the locking element or a housing for receiving a screw adapted to be screwed in a corresponding tapping in the locking element.
The first articulating surface has a concave generatrix, while the second and third articulating surfaces present, in a plane of section parallel to the axis of the first articulating surface, a convex generatrix. Thanks to this arrangement, the axis of the varus-valgus movement is permanently located in the ulnar component, which gives a better transverse stability to the prosthesis thus produced, in particular with respect to the one known by U.S. Pat. No. 4,242,758, in which the axis of the varus-valgus movement is, in the extreme configurations, located in the bobbin forming the humeral element.
The first articulating surface extends in a fourth, substantially convex, articulating surface for bearing a radial component. This allows a total elbow prosthesis to be produced.
A functional clearance between the above-mentioned surfaces is formed by the difference of their diameters or radii of curvature of their respective generatrices.
The ulnar component and/or the locking element comprise a reinforcement and a lining mounted on the reinforcement by cooperation of shapes thanks to returns adapted to cover lateral fins of the reinforcement.
An orifice for passage of at least one suture thread is made near the longitudinal axis of the first articulating surface and parallel thereto.